This invention relates to a magnetic tape cassette, and more particularly to a tape cassette of a structure with improved dimensional accuracy.
Cassettes of this character are generally manufactured by forming upper and lower housing sections (or halves) by injection molding, fitting a pair of tape-winding hubs, a length of magnetic tape wound on the hubs, guide rollers, a seperator sheet, magnetic head pad and spring, and other necessary components into the housing space, and then joining the two housing sections along their peripheral walls adapted to mate together by setscrews. The upper and lower housing sections are configured in a complementary manner so that when they are united all their portions combine to give a complete assembled structure as designed. Usually, the upper and lower housing sections of magnetic tape cassettes have symmetrically shaped front wall regions. FIGS. 1 and 2 illustrate the lower housing section of such a tape cassette having in the center of the front wall an opening 3 into which the magnetic head of a recorder not shown is to enter. Two more openings 5, 7 for pinch roller insertion are formed adjacent and symmetrically with respect to the head opening in the front center. One opening 5 is provided with a tape guide 9 generally U-shaped as viewed from above. The tape guide 9 includes guide ribs 9', 9" adapted to contact the tape. As shown in FIG. 2, the guide and ribs 9, 9', 9" have height greater than the width of the tape. At the rear of the magnetic head opening 3, there is formed a horizontally extending portion 11 of a given height, about halfway the height of the guide 9 to shield and protect the internal enclosure of the housing. The front wall of the upper housing section is likewise configured so that, when the upper and lower sections are joined together, the space behind the opening 7 is enclosed by the tape guide of the upper section (corresponding to the guide 9 of the lower section). Similarly, the back space of the center opening 3 is enclosed by the partition of the upper section (corresponding to the partition 11). There are also provided an opening 13 for a pinch roller, a positioning opening 15, and a pair of guide rollers 17.
The upper and lower housing sections of the construction described above frequently pose the problem of inadequate dimensional accuracy due to strains developed on injection molding. The inability of assuring constant dimensional accuracy is even more serious in these days when there is growing demand for higher performance recording and playback with magnetic tapes driven at lower speeds than heretofore.
Our investigations on the cause of the trouble have now revealed that the trouble stems from the fact that the back walls of the left and right openings adjacent the center opening have a substantial difference in height. At the rear of the left opening the guide 9 stands high, whereas the right opening is not backed by such a member. This can result in a material difference in friction resistance between the molded upper and lower housing sections at the time of their release from the dies. This dissimilarity in friction resistance, in turn, is likely to give unbalanced, defective moldings. A more important problem is a wide difference in mold shrinkage factor between the left and right openings which leads to permanent strains and reduced accuracy. The assembled product will have unbalanced strength between the left and right half portions, the right opening region being the weakest.
These configurations of the prior art have been possible causes of misalignment of the tape with the magnetic head of the recorder on which the cassette is used and hence of output drop or variation, phase mismatching, and other troubles.